Many metal halide discharge lamps are formed in a single-ended or single-based construction, in which a discharge vessel, typically of quartz glass, is provided, through which lead connections are carried; the discharge vessel is sealed by a pinch or press seal. To permit sealing the discharge connection through the pinch seal, external connecting wires or pins are secured to molybdenum metal foils which, in turn, are connected to the electrodes within the lamp. The pinch seal or press seal is formed in the region of the molybdenum foils, surrounding also the immediately adjacent portions of the connecting wires. The molybdenum foils extend parallel to each other, within the glass, and are located in a plane within the pinch or press seal, to be retained flat therein. In accordance with customary production techniques, the pinch seal is made in this manner:
(1) A circular cylindrical glass tube or cane is provided which, later on, will form the discharge vessel; an exhaust and fill gas supply and pump tube is connected thereto;
(2) a portion of the glass tubing, somewhat below the attachment of the pump tube, is heated to a temperature sufficient so that the glass of the tubing becomes plastically deformable;
(3) a subassembly of the electrode system, which includes the external current supply leads, the sealing foils, and the electrodes to be located within the discharge vessel, are introduced into the region of the glass tube which has been heated to plastically deformable temperature from the end remote from the pump tube; heating can also be done only after introducing the electrode system while holding it in a suitable holder;
(4) pinch jaws are then moved from diametrically opposite sides of the tubing towards the heated region of the tubing, the jaws being shaped to form dies or molds for the pinch seal to be made; the width of the pinch seal will be approximately the same as the diameter of the cylindrical tubing of the discharge vessel;
(5) the portion of the tube, now sealed at one end with the electrodes introduced, is evacuated and a fill is introduced through the pump tube, and the pump tube is tipped off.
In order to insure that the vessel is tight, that is, tightly sealed, it is customary to utilize sealing foils made of molybdenum in vessels in which the tubing is made of quartz glass. Continuous wire elements passing from the outside of the lamp into the lamp and carrying the lamp electrodes cannot be reliably sealed through quartz glass. The continuous trend towards miniaturization, and the demand for discharge lamps of lower output power, results in smaller sizes of the discharge vessels and, further, smaller sizes of the pinch seal, in which the sealing foils of molybdenum are embedded. Difficulties have arisen to maintain the required distance between the relatively wide sealing foils to be introduced through the pinch seal. These sealing foils are located in a single plane, and the maintenance of the spacing between the facing edges of the sealing foils, within the small and narrow pinch seal, is difficult. The electrodes have to be spaced from each other by some minimum spacing which is determined not by the operating voltage of the lamp but, rather, by the voltage of a voltage pulse which is necessary to cause the lamp to start or ignite. Depending on the temperature state of the lamp, an ignition voltage of at least 6 kV.sub.p must be applied to insure reliable starting.
In order to provide for the necessary insulation of the external current leads, it has previously been proposed to carry the current leads out from the pinch seal at an angle and to insulate the current leads from each other by glass tubes which are filled with a filling material and closed off by an elastic plastic substance (see British Pat. No. 1,442,497). This results in complex manufacturing steps which raise the price of the lamp out of proportion to the remaining manufacturing costs. The quartz insulating tubes which are comparatively long are subject to breakage and, in case they should break, the electrical insulation capability of the electrode leads is substantially impaired.
It has also been proposed to bend the sealing foils within the press seals by about 90.degree. towards the outside, so that the spacing of the external current supply leads is increased--see Japanese Utility Model Publication No. 47-308. Handling the sealing foils of molybdenum--which foils have a thickness of only a few hundredths mm (a few 10.sup.-5 m) thickness is very difficult and unsuitable. Reliability and durability upon passage of current through foils so treated is substantially reduced.
U.S. Pat. No. 3,617,797 describes an arrangement in which the current leads are carried out laterally from the press or pinch seal. This lamp is designed for use in an optical system, and the particular placement of the external connecting leads is arranged to permit the filament to have a predetermined distance from the end of the base; the press seal can be changed in its length subsequently to the original manufacture so that the position of the filament can be matched to the respective optical system.